1. Field of the Invention
The present invention is directed to an apparatus and process for fast, quantitative, non-contact topographic investigation of semiconductor wafers.
2. Discussion of Background Information
The microelectronics industry requires perfectly flat, mirror like surfaces having defect-free single-crystal wafers as a base for the production of integrated circuits and components. Any deviation from the ideal plane makes the manufacturing process difficult or even impossible or decreases the yield of the manufactured circuits. Such defects may often originate during the individual steps of the crystal and wafer production (cutting, polishing). Many of the technological phases of the production of the integrated circuits (annealing, layer deposition, patterning) may cause curving or warp of the originally flat surface. Consequently, the investigation of the flatness is crucial both for the wafer manufacturer and the consumer. Having a suitable investigating procedure, the wafers can be screened before using them, thus sparing many expensive technological steps. Not only the microelectronics industry requires the investigation of the mirror like surfaces; similar requirements have to be met for optical components, for some precision mechanical parts, as well as for the optical and magnetic disks of the IT industry. The investigating procedure requires high speed, non-contact operation, ability to investigate large-area (diam. >300 mm) samples, high sensitivity, and high lateral resolution (˜mm).
The requirement of the non-contact operation is met mainly by optical devices. In practice, the scanning laser beam and the interferometric methods are used. The scanning laser beam procedure uses a small-diameter parallel laser beam that scans the surface and from the position of the reflected laser beam the surface gradient of the actual surface point is determined, providing the surface topography. The disadvantage of the technique is its low speed, high cost and the need for high precision alignments. The interferometric procedures can measure only small-area surfaces.
A different principle is applied in the magic mirror technique (also called “Makyoh topography,” see, e.g., U.S. Pat. No. 4,547,073). This technique is depicted in prior art FIG. 1, and the principle of image formation is that a homogeneous parallel beam 2 falls on a surface 1 to be investigated. If the surface is perfectly flat, then a homogeneous spot appears on screen 3 positioned a certain distance away from surface 1. If surface 1 is not uniformly flat, the parallelism of the reflected beam is disturbed causing non-uniformity in its intensity distribution and an image appears on screen 3 that reflects the topography of surface 1. For example, a dent 4 focuses beam 2 causing an intensity maximum 6 on screen 3, while a hillock 5 defocuses beam 2 resulting in an intensity minimum 7. The sensitivity of the technique increases with increasing sample-screen distance. In practice, this basic set-up can be replaced by other, optically equivalent set-ups. For example, the collimated beam can advantageously be produced by a point source located in the focal point of a lens or a concave mirror. The beam reflected from the sample can pass through the lens or can be reflected from the concave mirror and the image can appear on a CCD camera. With suitable set-up, the sensitivity of this method meets the strictest requirements of the semiconductor industry, e.g., detection of a 0.05 μm deep surface dent over a 0.5 mm distance has been reported. However, the disadvantage of this method is the lack of the quantitative evaluation.
International Publication No. WO 00/29835 discloses a completed set-up, as described above, by taking two pictures at two different sample-screen distances; the surface topography and reflectivity map was determined by the iteration of the diffraction integrals of the surface. The method can provide quantitative results, but the disadvantage is the extreme slowness of the algorithm and the high requirements concerning the quality of the beam and the mechanical adjustments.
Prior art FIG. 2 illustrates a set-up described that is similar to the magic mirror arrangement [see K. H. Yang, Journal of the Electrochemical Society, Vol. 132. p. 1214. 1985]. In this set up, a light beam collimated by a collimator 1 falls to surface 3 and a reflected image is formed on screen 4 located some distance away from surface 3. The illuminating beam 1 traverses a quadratic grid 2, and from the position of the image of the grid points, a suitable algorithm calculates the curvature of the surface. The reported evaluation method is suitable only to determine uniform curvatures. A further disadvantage is that, as a consequence of the great grid-sample and grid-screen distances, the diffraction effects cause blurring of the image of the grid, which results in an inaccurate determination of the grid points. Thus, the error of the method increases. Moreover, greater deformation may cause an overlap of the image of the grid points that inhibits evaluation, and limits the density of the grid points decreasing the achievable lateral resolution. The non-normal incident angle causes additional distortions. Another serious disadvantage is the great size of the set-up (several meters).
The Hartman test is known for the evaluation of optical components, especially astronomic mirrors of large diameter, by projected masks [see Optical shop testing, ed. D. Malacara, John Wiley and Sons, New York, 1978, p. 323.] A typical realization of the technique is shown in prior art FIG. 3, in which the light of point source 1 is projected to surface 3 to be investigated through a mask 2, which is an opaque plate with holes, and the beam reflected through the holes reaches screen 4. From the position of the reflected beam of a given (x,y) point on screen 3, the height of the point h(x,y) compared to a reference point having an arbitrarily chosen height of zero, can be calculated by the summation approximation of an integral where the summation is carried out between the reference and the given point on the route defined by the neighboring holes of mask 2. The members of the summation are the product of three quantities: a geometrical constant characteristic to the optical lay-out, the difference of the measured coordinates of the ideally flat and the real surface, and the distance between the given and the neighboring points. For example, for quadratic grids the calculation can be carried out by the equation:
      h    ⁡          (              x        ,        y            )        =            1              2        ⁢        L              ⁢                  ∑        i            ⁢              [                              Δ            ⁢                                                  ⁢                          x              ⁡                              (                                                      x                    i                    ′                                    -                                      f                    xi                                                  )                                              +                      Δ            ⁢                                                  ⁢                          y              ⁡                              (                                                      y                    i                    ′                                    -                                      f                    yi                                                  )                                                    ]            where L is the geometric constant, Δx and Δy are the lengths of the grid projected on the sample surface, (fxi, fyi) are the measured coordinates of the image of the surface point (xi, yi) and (xi′, yi′) are the coordinates of the image of the point (xi, yi) for an ideal flat surface. In the practice, more accurate but essentially not different integral approximations can be used.